This is the best answer for which set of u.s. presidents have descendants who are married to each other?
The set of U.S. Presidents that have descendants who are married to each other are Richard Nixon and Dwight Eisenhower.
President Eisenhower's only grandson, David Eisenhower, married President Nixon's daughter Julie Nixon in 1968. The couple have three children, Jenny Elizabeth, Alexander Richard and Melania Catherine Eisenhower, President Nixon's grandchildren and grandchildren. - Children of President Eisenhower.